1. Field of the Invention
The present invention relates to a method of manufacturing a silicon nitride film.
2. Description of the Related Art
Conventionally, silicon nitride (Si.sub.X N.sub.Y) is a solid and chemically inert dielectric material having a very high hardness, a low thermal conductivity, and high resistance to molecular diffusion. Due to these properties, silicon nitride is used as an oxidation mask, a gate insulating film, a passivation film, or the like in, e.g., the manufacture of a semiconductor device. A method of forming a silicon nitride film on a substrate, e.g., the surface of a semiconductor substrate is disclosed in Published Examined Japanese Patent Application No. 60-10108. According to this method, a substrate is arranged in a reaction tube, a gas mixture of dichlorosilane and ammonia flows in the reaction tube, and the substrate is brought into contact with the gas mixture at a low pressure and a high temperature of 700.degree. C. to deposit a silicon nitride film on the substrate by thermal decomposition. The reaction of dichlorosilane and ammonia is given by the following formula: EQU 3SiH.sub.2 Cl.sub.2 +4NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +6HCl+6H.sub.2
In order to obtain a high growth rate of a film, uniform deposition, and a high-quality film in an economical process, ammonia must flow in the reaction tube at a gas flow rate five times that of dichlorosilane.
However, since excessive ammonia flows in the reaction tube, non-reacted ammonia remains in the reaction tube. As is apparent from the above reaction formula, hydrochloric acid produced as a reaction product is reacted with the excessive ammonia to produce ammonium chloride. Although ammonium chloride is not solidified at a temperature of 150.degree. C. or more, ammonium chloride is deposited in a solid state at a temperature of less than 50.degree. C. For this reason, since portions in the reaction tube or tubes coupled thereto have a temperature of less than 150.degree. C., e.g., portions located on a downstream side from gas supply and exhaust ports in a normal low-pressure CVD (Chemical Vapor Deposition) furnace have a temperature of 150.degree. C. or less, ammonium chloride is deposited on these portions. The ammonium chloride deposited as described above is removed from the inner wall of the reaction tube when the pressure in the reaction tube is changed from the atmospheric-pressure state to a low-pressure state or changed from the low-pressure state to the atmospheric-pressure state, and the ammonium chloride is present as particles in the reaction tube. Therefore, the particles are attached on the substrate surface. In addition, when ammonium chloride is attached on a metal portion exposed to the reaction region and having a relatively low temperature of 150.degree. C. or less, the metal portion on which the ammonium chloride is attached is corroded by moisture remaining in the reaction tube due to loading or unloading of the substrate. For this reason, the reaction tube must be periodically washed or heated to remove ammonium chloride.
In order to prevent deposition of ammonium chloride, silane (SiH.sub.4) may be used in place of dichlorosilane. In this case, however, since it is difficult to control a deposition speed of silicon nitride, the thickness of a silicon nitride film formed on the surface of the substrate is difficult to uniformize. For example, a thicknesses distribution of a silicon nitride film is not uniform in each wafer, or silicon nitride films having the same thickness cannot be obtained on a plurality of wafers.